Electrically-programmable, read-only memories (EPROMs) and electrically-programmable, electrically-erasable read-only memories (EEPROMs) are non-volatile semiconductor memory devices based on metal oxide semiconductor field effect transistors (MOSFETs). EPROM and EEPROM cells store a bit of information as a quantity of electrons on a floating gate structure insulatively formed between the channel and the control gate of the field effect transistor. A charged floating gate raises the threshold voltage of the field effect transistor channel above the voltage normally applied to the control gate during the read operation such that the transistor remains shut off when read voltages are applied to the gate, source and the drain, thereby returning a logic "zero." An uncharged floating gate does not alter the threshold voltage of the channel of the field effect transistor, and therefore a normal gate reading voltage will exceed the threshold voltage, turning on the transistor when read voltages are applied to the gate, source and the drain, and thereby returning a logic "one."
One structure used in EPROM cells and some EEPROM cells is the floating gate avalanche injection metal oxide semiconductor (FAMOS) structure. In this structure, the floating gate is charged by avalanche injection, commonly referred to as "hot electron injection". The majority of the electrons are normally injected to the floating gate from the channel near the drain region of the cell. This process, known in the art as "drain side injection," is a function of the voltages applied to the source and the drain regions and to the control gate, the source being held at a voltage lower than that applied to the drain during programming.
In drain side electron injection, the source is normally grounded or held close to zero volts while the drain is pulsed with a positive voltage. Application of a relatively high voltage pulse to the control gate induces an inversion layer in the channel area of the semiconductor between the source and the drain, allowing electrons to flow from the source to the drain. Due to the interaction between the electric field across the gate oxide created by the voltage applied to the control gate and the electric field in the channel created by the voltage applied to the drain region, the inversion layer near the drain is reduced in depth or "pinched-off." In this "pinch-off" region the electric field is enhanced and the electrons are accelerated towards the drain. A number of these accelerated electrons collide with the semiconductor lattice in the channel, creating electron-hole pairs. An avalanche effect results as the newly created electrons are themselves accelerated through the pinch-off region causing more collisions with the lattice. Some of the electrons created during this process ("hot" electrons) have a sufficient energy level to jump over the potential barrier created by the insulator between the floating gate and the channel. A percentage of these "hot" electrons will in turn be drawn to the floating gate because of the voltage induced on it by capacitive coupling with the control gate. These electrons come to reside on the floating gate, thereby charging it.
Conventional drain side electron injection has serious drawbacks. To simultaneously generate hot electrons and achieve sufficient gate current requires that the drain voltage and the gate voltage both be high. Drain side electron injection biasing does not favor this condition. Further, undesired charging of the floating gate can occur anytime a voltage is applied to the drain, such as when the drain is part of a bitline in an array of memory cells. These disadvantages can be eliminated if the injection of electrons can be moved towards the source side of the channel, and if electric field on the drain side can be reduced during normal read and write bias condition, for the same conventional application of voltages.
Further, if the hot electron injection can be moved to the source side of the cell, a much more significant gate current can be realized since the electric field across the gate oxide near the source, as created by the voltage difference between the grounded source and the control gate is at a maximum. Those electrons resulting from the collision/avalanche process have a high probability of being injected to the floating gate due to the high electric field. This makes a 5-V only non-volatile memory device possible. Additionally, erase times can be decreased if the floating gate overlap of the drain is increased and the electric field near the drain is decreased.
Thus a need has arisen for a nonvolatile memory cell using hot electron injection programming in which the injection process has been moved towards the source area, and in which the maximum electric field on the drain side can be reduced. Such a device will take advantage of the maximum electric field available for a given control gate voltage which will ultimately allow for a reduction in control gate voltage actually required. The cell will also eliminate problems resulting from unwanted injection of electrons onto the floating gate when a voltage is applied to the drain, such as when the drain is formed as part of a bitline for a column of cells in a memory array.